Fix crash when pressing 'u'. Thanks to Quentin Carbonneaux.

ListItem.c

@@ -45,7 +45,8 @@ static void ListItem_display(Object* cast, RichString* out) {
 
 ObjectClass ListItem_class = {
    .display = ListItem_display,
-   .delete = ListItem_delete
+   .delete = ListItem_delete,
+   .compare = ListItem_compare
 };
 
 ListItem* ListItem_new(const char* value, int key) {